1. Field of the Invention
The present invention relates to rechargeable batteries, and more particularly relates to rechargeable alkaline MnO.sub.2 --Zn battery cells having improved cycle life.
2. Background Information
Manganese dioxide has been used extensively in primary, single-use battery cells and particularly in dry and alkaline manganese dioxide-zinc cells. These battery cells are generally made in a charged states, are used once and are then discarded.
Recently, in view of both expanding energy requirements and environmental concerns about proper waste disposal of used battery cells, great emphasis has been placed upon the development of readily rechargeable battery cells which can be used in a wide range of equipment and consumer products. Rechargeable batteries would be useful in many applications such as electric vehicles and consumer electronic products. Examples of other areas particularly suited to the use of rechargeable batteries include satellites, space travel equipment and as backups for solar energy systems.
It is desirable to continue to use manganese dioxide in batteries because of its low cost and its stability. Unlike other rechargeable batteries, manganese-oxide/zinc batteries do not generally contain toxic components. The commonly used form of manganese dioxide itself is not readily rechargeable. Furthermore, rechargeability is essentially impossible if the material has been deep discharged to greater than 50 percent of the theoretical two-electron capacity.
Various attempts have been made at producing rechargeable compounds containing manganese dioxide. Kordesch, et al., Electrochemica. Acta, 26, 1495 (1981), discloses that in certain circumstances electrolytic manganese dioxide may be recharged in the range of 100 times. However, this is only true if the material is discharged to less than 30 percent of the theoretical one-electron capacity, which is equivalent to less than 15 percent of the theoretical two-electron capacity. This restriction requires shallow discharge which severely limits the amount of energy which can be retrieved from the cell. It also reduces the rechargeability or eliminates rechargeability entirely if the manganese dioxide is accidentally discharged beyond the limit. Thus, there is very little practical application to the rechargeability.
Kordesch, Journal of Power Sources, Vol. 51, p. 61-78 (1994), discloses an alkaline battery comprising manganese dioxide having limited rechargeability. The battery includes a relatively large amount of MnO.sub.2 cathode material in comparison with conventional throw-away cells, and is designed to avoid the formation of Mn(OH).sub.2. Due to nonuniform discharge of the manganese dioxide cathode, the recharge capacity of the cell drops approximately exponentially from cycle to cycle, resulting in an unsatisfactory life of only about 10 to 20 cycles.
U.S. Pat. No. 4,520,005 to Yao discloses a chemical process to prepare a compound which includes manganese dioxide doped with bismuth and/or lead. The resulting compound is rechargeable, however, its method of preparation is a complex and slow batch process. Furthermore, the compound has lower density than is desirable in many commercial applications. Density can be a critical factor in battery electrode materials because of volume limitations in battery cells. In a given volume, a more dense material produces more energy than would a material of lower density such as the material disclosed in U.S. Pat. No. 4,520,005. The low density powder with poor electronic conductivity requires the addition of large quantities of graphite (about 90 weight percent) to make enough electronic contact to render the mixture sufficiently electronically conducting and rechargeable. After optimizing the overall capacity with this high-graphite mixture in a C-size cell, the cell would only have a maximum capacity of about 1.2 Ah based on discharge and charge to 80 percent of two electrons. This low capacity is impractical for a competitive C-cell. Such cells discharging at 1.0 V, even at high rate, are also impractical replacements for alkaline primary cells discharging at an average of 1.25 V, and nickel-cadmium or nickel-hydrogen secondary cells discharging at 1.2 V, since they will not match the electrical requirements of typical loads.
The background research which led to the development of the compounds described in U.S. Pat. No. 4,520,005 has been described in Yao et al., Journal of Electroanalytical Chemistry, 223, 107 (1987). The objective was to use an MnO.sub.2 starting material with a modified lattice which would not collapse when the Mn.sup.2+ reduction stage was reached. Hydrated compositions known as birnessites and buserites contain sheets of H.sub.2 O and OH.sup.- trapped between two-dimensional sheets of edge-shared Mn.sup.4+ O.sub.6 layers, which in naturally-occurring compounds are held together by anions such as Na.sup.+. The authors attempted to show that incorporation of cations into these layers, e.g., by ion-exchange after preparation of the layered birnessite-type compounds, could prevent ultimate lattice collapse on reduction to Mn.sup.2+, provided that the exchanged ions remained in the lattice. The ions of a large number of metallic elements were examined as possible substituents, but with the exception of bismuth and lead, none conferred adequate rechargeability after complete reduction. It is also of interest that Mn(OH).sub.2 compositions doped with Al.sup.3+, Ca.sup.2+, Ba.sup.2+ and Mg.sup.2+ were examined by Bauer et al., Ber. Bunsenges. Phys. Chem. 90, 1220 (1986). Only slight rechargeability of MnO.sub.2 was observed by Bauer et al., the Ba.sup.2+ -doped Mn(OH).sub.2 being rechargeable to the extent of about 91 mAh/g, or 15% of the theoretical 2-electron capacity. This is too low to be practical.
As noted above, the two metals whose ions Yao et al. found did give close to the theoretical two-electron capacity when ion-exchanged into MnO.sub.2 compounds with the birnessite lattice were bismuth and lead, or mixtures thereof. U.S. Pat. No. 4,520,005 indicates how such compounds may be made. As stated above, their lack of utility in secondary batteries make them of limited practical application. Another problem observed with these bismuth and/or lead doped birnessite materials is their sensitivity to the presence of dissolved zinc as Zn(OH).sub.4.sup.2- in alkaline electrolytes when attempts were made to use them in secondary MnO.sub.2 --Zn cells. This was discussed by Dzieciuch et al., Journal of the Electrochemical Society, 135, 2415 (1988). This paper gave evidence for the formation of a stable mixed Zn--Mn.sup.3+ oxide, heterolite, which could not be further charged. In this paper and in U.S. Pat. No. 4,451,543, Dzieciuch et al. described a secondary MnO.sub.2 --Zn cell using a modified bismuth and/or lead doped birnessite cathode material, in which methanol was added to the 9 molar KOH electrolyte to suppress the solubility of Zn(OH).sub.4.sup.2- from greater than 1 molar to below approximately 0.1 molar. This resulted in improved cyclability, 25 percent of the theoretical 2-electron capacity of the positive electrode being retained after 100 cycles, which was maintained for a further 100 cycles. However, the positive electrode contained only 8.3 weight percent of birnessite material, the remainder being graphite containing 13.8 weight percent of acetylene black. In an optimized C-cell, the practical capacity after 100 cycles would only be 0.4-0.5 Ah, which is only about 20 percent of the capacity of a conventional Ni--Cd secondary cell of the same dimensions.
In another prior art approach, manganese dioxide is physically mixed with oxides of bismuth, lead or mixtures thereof (Wroblowa, et al., Journal of Electroanalytical Chemistry, 238, 93 (1987)). An electrode made in accordance with this method including manganese dioxide doped with bismuth oxide was shown to be rechargeable up to 250 cycles, but with a continuous drop in capacity as the number of cycles increased. In addition, the physical mixing process is time consuming and may lead to non-homogeneous mixtures, thereby affecting the performance of the electrodes. Moreover, the electrodes prepared with physical mixing require several electrochemical cycles for activation and use. The resulting material is not suitable for sealed cells because of the problem of gas evolution. In cells incorporating commercially-available battery separators, the slightly-soluble bismuth compound finds its way to the zinc anode, where bismuth metal deposits, giving a high-area bismuth black surface on which gaseous hydrogen is evolved. This results in self-discharge, with the accompanying danger of mechanical failure of the sealed cell.
In another method disclosed in U.S. Pat. No. 5,156,934 to Kainthla et al., which is incorporated by reference herein, commercially available electrolytic manganese dioxide or commercially available chemical manganese dioxide is used. The particles of the prepared electrolytic or chemical manganese dioxide are coated with bismuth hydroxide or lead hydroxide. The electrode prepared with the bismuth-coated product shows good rechargeability and sufficient density and conductivity for commercial application. However, the material prepared in accordance with this method has limited usefulness in sealed battery cells. Typically, a zinc anode and commercially available separator materials would be used. When using separator materials in alkaline batteries having potassium hydroxide as an electrolyte, gassing has been found to occur. This gassing is due to a reaction which occurs involving the unreacted bismuth oxide or bismuth hydroxide in the cell. A finite amount of the bismuth oxide or hydroxide is soluble in the alkaline electrolyte, which passes through the commercial separator to the anode compartment and deposits on the zinc electrode. The resultant gas buildup within the cell can present a problem in sealed cells.
U.S. Pat. No. 5,419,986 to Kainthla et al., which is incorporated by reference herein, discloses the preparation of a compound having improved electronic conductivity in comparison with the material described in U.S. Pat. No. 4,520,005. The material disclosed in U.S. Pat. No. 5,419,986 has relatively high density and can be electrochemically cycled in mixtures containing up to 90 percent of active material and 10 percent of a mixture of suitable graphites and carbons. A further advantage of the disclosed compound is that it is insensitive to the presence of dissolved Zn(OH).sub.4.sup.2-, in the electrolyte, as has been demonstrated by operating an electrode of this material containing a zinc electrode against a nickel counter-electrode. The nickel electrode operates on either charge or discharge in a regime of electrochemical potential in which it is thermodynamically impossible for zinc to deposit, so zinc remains as Zn(OH).sub.4.sup.2- in the cathode area. Since the material cycles well even in the presence of Zn(OH.sub.4).sup.2-, preventing Zn(OH).sub.4.sup.2- transport from the anode to the cathode is not a problem with the type of modified manganese dioxide described in U.S. Pat. No. 5,419,986. Whereas other investigators indicate that rechargeability of certain modified MnO.sub.2 compounds results from doping of birnessite-type lattices, it has been determined that this explanation is not the reason for the rechargeability of the modified MnO.sub.2 compound prepared according to U.S. Pat. No. 5,419,986.
Despite the foregoing efforts, a need still exits for a rechargeable MnO.sub.2 --Zn battery having significantly improved life cycle.